Dinitrotoluene is usually prepared by nitrating toluene with a mixture of nitric and sulfuric acids ("mixed acid"), as described in "The Chemistry of Powder and Explosives," by T. L. Davis, pg. 148, in "Methoden der Organischen Chemie (Houben-Weyl), " Vol. X/1, pg. 525, and in Interscience Encyclopedia, Vol. 13, pg. 851. This procedure involves the removal of water by the use of sulfuric acid, the recovery of which adds to the cost and presents a pollution and disposal problem. The elimination of sulfuric acid in nitration with azeotropic removal of the water by heating the hydrocarbon, or partially nitrated hydrocarbon, with a diluent acting as an entraining agent for the water has been described by Kokatnur in U.S. Pat. Nos. 2,435,314 and 2,435,544 (1948). The mononitration of toluene by this method has been studied by Othmer and Kleinhans (Ind. Eng. Chem. 36,477 (1944)). A somewhat similar approach was used by Crater (U.S. Pat. No. 2,362,743 (1944)), who mononitrated toluene with 70% nitric acid, separated the resulting mononitrotoluene, and then converted it to dinitrotoluene by heating it with 98% nitric acid at 80.degree. C.
It is known to produce high-strength (98%) nitric acid by the reaction of nitrogen dioxide and oxygen with water in the presence of nitric acid according to the following reaction: EQU 2 NO.sub.2 + 1/2 O.sub.2 + H.sub.2 O.fwdarw. 2 HNO.sub.3
there are several commercial procedures for doing this, including those of Fauser (Chem. Eng. 59(1), 238 (Jan 1952)), Peroxide (Chem. Eng., Dec. 25, 1972, pg. 50), Sumimoto (Nitrogen, No. 74, 40 (1971)), Hycon (Nitrogen, No. 79, 24 (1972)), and Sabar (European Chem. News, Aug 24, 1973, pg. 27). These procedures all employ pressure, varying from 8 atm. (Peroxide) to 52 atm. (Fauser). The Fauser process operates at 70.degree.-75.degree. C. and uses oxygen; the Peroxide process employs air.